Mon, Feb 10, 2020 - Page 7 News List

As forests burn around the world, drinking water is at risk

More than 60 percent of the water supplying the world’s 100 largest cities originates in fire-prone watersheds, where rain can wash huge volumes of ash, sediment and debris into crucial waterways and reservoirs

By Tammy Webber  /  AP

Illustration: Yusha

Fabric curtains stretch across the huge Warragamba Dam to trap ash and sediment expected to wash off wildfire-scorched slopes and into the reservoir that holds 80 percent of untreated drinking water for the greater Sydney area.

In Australia’s capital, Canberra, where a state of emergency was declared Jan. 31 because of an out-of-control forest fire to its south, authorities are hoping a new water treatment plant and other measures will prevent a repeat of water quality problems and disruption that followed deadly wildfires 17 years ago.

There has not yet been a major effect on drinking water systems in southeast Australia from the intense fires that have burned more than 104,000km2 since September last year, but authorities know from experience that the biggest risks are to come with repeated rains over many months or years, while the damaged watersheds, or catchment areas, recover.

Due to the size and intensity of the fires, the potential effects are also not yet clear.

“The forest area burned in Australia within a single fire season is just staggering,” said Stefan Doerr, a professor at Swansea University in Wales who studies the effects of forest fires on sediment and ash runoff. “We haven’t seen anything like it in recorded history.”

The situation in Australia illustrates a growing global concern: Forests, grasslands and other areas that supply drinking water to hundreds of millions of people are increasingly vulnerable to fire due in large part to hotter, drier weather that has extended fire seasons, and more people moving into those areas, where they can accidentally set fires.

More than 60 percent of the water supply for the world’s 100 largest cities originates in fire-prone watersheds — and countless smaller communities also rely on surface water in vulnerable areas, researchers have said.

When rain does fall, it can be intense, dumping a lot of water in a short period of time, which can quickly erode denuded slopes and wash huge volumes of ash, sediment and debris into crucial waterways and reservoirs.

Besides reducing the amount of water available, the runoff can also introduce pollutants, as well as nutrients that create algae blooms.

What is more, the area that burns each year in many forest ecosystems has increased in the past few decades, and that expansion is likely to continue through the century because of a warmer climate, experts have said.

Most of the more than 64,000km2 that have burned in Victoria and New South Wales have been forest, including rainforests, scientists in New South Wales and the Victorian government have said.

Some believe that high temperatures, drought and more frequent fires might make it impossible for some areas to be fully restored.

Very hot fires burn organic matter and topsoil needed for trees and other vegetation to regenerate, leaving nothing to absorb water. The heat also can seal and harden the ground, causing water to run off quickly, carrying everything in its path.

That can clog streams, killing fish, plants and other aquatic life necessary for high-quality water before it reaches reservoirs.

Already, thunderstorms in southeast Australia in the past few weeks have caused debris flows and fish kills in some rivers, while fires continue to burn.

“You potentially get this feedback cycle,” where vegetation cannot recolonize an area, which intensifies erosion of any remaining soil, said Joel Sankey, research geologist for the US Geological Survey.

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